Revisiting the Alfven-Parker Debate

In this talk, we will discuss the debate over the use of E = -VxB (MHD paradigm) to describe space plasmas. Alfven (Rev. Geophys. Space Phys., 15, 271, 1977) cautioned against applying this ideal equation to collisionless plasmas on the grounds that fluid description is then not valid. However, Parker (J. Geophys. Res., 101, 10,587, 1996) has incorporated this ideal equation in the MHD fluid theory and argues that his formulation effectively describes space plasma dynamics. To resolve this debate, we turn to first principles. Electric fields in moving plasma and stationary spacecraft frames are related by Epl = Esc + VxB. With Epl = 0, the total magnetic flux is conserved (df/dt = 0) and the electromotive force (EMF) is not induced. Hence, currents and magnetic fields cannot be generated by the plasma and Joule heating vanishes (J* E = 0). Our conclusion is that these conditions imposed by the MHD paradigm restrict Parker's theory to describing only the dynamics of non-dissipative space plasmas that have preexisting currents and magnetic fields. However, inductive and EMF effects are integral to plasma processes that require formation of new currents and magnetic fields (flares, auroras, reconnection, shocks, magnetic storms and substorms). For the physics to be self-consistent, these dynamical problems must be examined with non-vanishing Epl, no matter how small Epl is.